This invention relates to optically mismatched etalons and optically mismatched stacked, optically coupled etalons and to methods of making and using them, as well as to devices incorporating such optically mismatched etalons and optically mismatched stacked, optically coupled etalons.
Etalons are ubiquitous in optical systems, such as optical sensors, optical communication systems, etc. The basic Fabry-Perot etalon can be designed and produced to have a sharp response at resonant frequencies, which makes them suitable as optical filters such as bandpass filters. They also give a variable amount of dispersion, and so have been suggested for possible use as dispersion compensators. Thus Fabry-Perot etalons are a basic building block in a number of different optical elements, i.e., in optically functional components or devices. Such devices may be active or passive and may be employed in a system (or adapted to be employed in a system) to pass or transmit a selective wavelength or band of wavelengths or periodic set of wavelength bands. Exemplary optical elements in which etalons are used include optical sensors, and filters, e.g., band pass filters, single channel filters, and other wavelength selective filter devices such as wavelength division multiplexers, and dispersion compensators and other components of optical communication systems.
Etalons typically comprise precisely parallel selectively transmissive surfaces such as thin films, i.e., partially reflective mirrors or surfaces on opposite sides of an integral number of half waves distance or gap between them, forming the etalon""s cavity. The thin film and cavity characteristics determine the optical properties of the etalon. That is, the spectral characteristics of the etalon are generally determined by the reflectivity of the mirrors or surfaces and by the optical thickness of the cavity length. Such etalons have long been produced, for example, by sputter deposition of film stacks of alternating layers of materials, i.e. a high refractive index material alternating with a low refractive index material, to form a mirror coating, which is transmissive of selected wavelengths. Two such mirror coatings sandwich a sputter-deposited cavity layer between them. Sputtering or other physical vapor deposition of the relatively thick cavity layer is time consuming and, therefore adds substantial time and cost to the production of such etalons. The result is undesirably high cost for production for such etalons.
It has long been a recognized problem in this industry, that producing etalons having desired properties can be difficult and expensive. In addition, there are industry-recognized problems associated with producing structurally robust etalons having desired, precise optical properties. Prior known etalons have employed various designs, such as the etalons used in the interferometric optical devices of U.S. Pat. No. 6,125,220 to Copner et al. In the interleaver/de-interleaver devices of Copner et al, two glass interferometric end plates are separated by a spacer region where the etalon is formed. The spacer region is an air gap having a predetermined dimension. In adjustable Fabry-Perot devices, such as those disclosed in U.S. Pat. No. 5,283,845 to Ip, tuning of the center wavelength of the spectral passband of an etalon is achieved by varying the effective cavity length (spacing) between two end plates carrying thin film reflectors. More specifically, in Ip a piezo actuator is used, extending between the two end plates. By varying the electric power applied to the piezo actuator, the axial length of the actuator can be varied, and thus the gap between the end plates varied. As alternatives to piezo-electric actuators, the tuning mechanism may include liquid crystals, temperature, pressure, and other mechanisms. It is a disadvantage that adjustable etalons as in Ip involve considerable assembly complexity and cost. Also, maintaining strict parallelism between the end plates can present additional difficulties.
The prior known optical etalons, as noted above, fail to fully meets the needs of many applications, especially for optical elements intended for optical communication systems, precision sensors, etc.
It is an object of the present invention to provide optical filter elements comprising optically mismatched etalons and optically mismatched stacked, optically coupled etalons addressing some of the deficiencies of the prior known technologies. It is a particular object of at least certain preferred embodiments, to provide optically mismatched and directly optically coupled etalons and optically mismatched and directly optically coupled and stacked, optically coupled etalons and methods of making same, and optical systems incorporating such optically mismatched etalons and optically mismatched stacked, optically coupled etalons. Additional objects and aspects of the invention and of certain preferred embodiments of the invention will be apparent from the following disclosure and detailed description.
In accordance with a first aspect, an optical filter element is provided for filtering multiplexed light, comprising multiple directly optically coupled etalons, at least a first and a second of the etalons having optically mismatched periodic passbands, as more fully described below. In accordance with another aspect an optical system comprises a source of light, preferably multiplexed light, and an optical filter element as just described. Multiplexed light, as used here, is light having multiple channels, for example 1-n channels where n is the total number of channels. In preferred embodiments, an optical filter element comprises at least first and second Fabry-Perot etalons, preferably a plurality of Fabry-Perot etalons, that are optically mismatched, e.g. have optically mismatched periodic passbands, and that are directly optically coupled. As used here, optically mismatched, optical mismatching, or optically mismatching occurs when etalons having a different passband response are placed into an optical element. As used here, optically mismatched periodic passbands will be understood by those skilled in the art, given the benefit of this disclosure, to be mismatched sufficiently such that only one the passbands of a first etalon (or of the first etalon stack, as the case may be) overlaps any passband of the second etalon (or of a second etalon stack, as the case may be) within the wavelength range of interest. In a typical optical filter element disclosed here, the passbands of the first and second etalons may have a wavelength width approximately equal to the width of a single channel of multichannel multiplexed light. For example, in a telecommunication system operating in the C-band (approx 1530 nm to 1570 nm), wherein the C-band is divided into 40 channels, the channels each would be about 0.8 nm, and the passbands for each channel typically would be about 0.4 nm and centered in the allotted 0.8 nm channel. The period of the two etalons would, of course, as disclosed above, be sufficiently different that they overlap each other only once in the C-band. As used here, passbands overlap if they have approximately the same center wavelength and/or together pass sufficient light or signal strength for the common passband to be operative as a channel of the multichannel system in which the optical filter element is employed. Therefore, the optical filter element can act to filter out all passbands except the overlapping passband. Optical components, such as etalons and stacked, optically coupled etalons, can be directly optically coupled, as the term is used here, when they are optically coupled, i.e. are in the same optical path, and furthermore are in optical contact or are otherwise in physical contact with each other and/or mounted to each other (e.g. by bonding material in or out of the optical path) or mounted together in the same housing or by the same fixture. An air space may be separating the etalons, or the stacked, optically coupled etalons, or the etalons, or stacked, optically coupled etalons, may be in direct surface-to-surface contact. Similarly, optical components in an optical system comprising the etalons, or stacked, optically coupled etalons, disclosed here, that are directly optically coupled in accordance with the present disclosure, preferably have no intervening optical components performing substantial channel filtering or like optical operations on any passed signals. In particular, any signals passed by the etalons, or stacked, optically coupled etalons, or other optical components in the optical system, arrive at a second etalon, or second stacked, optically coupled etalons, or other optical component, without any intervening wavelength filtering optical operations to add or drop passbands or like operations. One skilled in the art, given the benefit of this disclosure, will be able to design and assemble optical systems comprising directly optically coupled etalons, directly optically coupled and stacked, optically coupled etalons, and other optical components described here.
In accordance with another aspect, an optical filter element, suitable for use in an optical system, comprises stacked optically coupled etalons, wherein the etalons of the stack have different passband responses. Each of the etalons, of the stacked, optically coupled etalons, may be placed in optical contact, i.e. the thin film coatings of a first etalon in direct and substantially continuous surface-to-surface contact with the thin film coatings of an adjacent etalon in the stack, or may be placed in contact using one or more bonding layers optically coupled with each other between adjacent etalons. As used herein, a bonding layer is any layer of bonding material on a surface of an etalon and used to physically attach that etalon to an adjacent etalon. The bonding layer optionally is in the light path through the stacked etalon and serves also to optically couple the adjacent etalon. In such embodiments, preferably the thickness of the bonding layer is equal to an odd number of quarter wavelength optical thickness (QWOTs). This bonding layer may comprise an adhesive, such as an epoxy, e.g. the epoxies available from Epoxy Technology, Billerica, Mass., such as EPO-TEK 353 ND, an adhesive optionally having a monolayer of small beads embedded in the adhesive to facilitate precise spacing of the adjacent etalons, fritted glass or any other composition or material that may be deposited to an odd number of QWOTs and is capable of bonding the etalons together. The stacked, optically coupled and optically mismatched etalons can be housed within an optical system. Preferably, the first etalon in the stacked, optically coupled and optically mismatched etalons is selectively transparent to only certain passbands. The second etalon in the stacked, optically coupled and optically mismatched etalons is also selectively transparent to only certain passbands, and the passband response of the second etalon differs from the passband response of the first etalon. In accordance with preferred embodiments, only a single passband of the first and second etalons overlap. Therefore, stacked, optically coupled etalons comprising optically mismatched etalons can be used as an optical filter to select only a single passband.
In accordance with additional aspects, an optical filter element, suitable for use in an optical system, comprises at least a first etalon optically coupled and optically mismatched with at least first stacked, optically coupled etalons. Preferably the first etalon comprises selectively transparent thin film coatings that act to pass only certain passbands. Preferably the first stacked, optically coupled etalons also selectively pass only certain passbands, and the passband of the first stacked etalons only overlaps the passband of the first etalon at a single passband. The first etalon and first stacked, optically coupled etalons can be housed within an optical system, and the first etalon and first stacked, optically coupled etalons may be separated by air space or may be in direct contact. Directly optically coupling the first etalon and first stacked, optically coupled etalons can result in the filtering of all passbands of multiplexed light except for the one overlapping passband of the etalon and stacked, optically coupled etalons. Therefore, optically mismatching of an etalon and stacked, optically coupled etalons can be used as an optical filter to select only a single passband.
In accordance with another aspect, an optical filter element, suitable for use in an optical system, comprises at least first and second stacked, optically coupled Fabry-Perot etalons that are optically mismatched. In accordance with preferred embodiments, each stack of etalons comprises optically matched etalons. However, different stacked, optically coupled etalons may be optically mismatched to select a passband from multiplexed light. Each of the etalons, of the stacked, optically coupled etalons, may be placed in optical contact, i.e. the thin film coatings of a first etalon in direct and substantially continuous surface-to-surface contact with the thin film coatings of an adjacent etalon in the stack, or may be placed in contact using one or more bonding layers optically coupled with each other between adjacent etalons. The first and second stacked optically coupled etalons may be separated by air space or may be in direct contact. The first and second stacked, optically coupled etalons can be housed within an optical system. In accordance with preferred embodiments, the first stacked, optically coupled etalons are selectively transparent to only certain passbands, and the second stacked, optically coupled etalons are also selectively transparent to only certain passbands, preferably different passbands except for one overlapping passband. That is, the passbands of the first and second stacked, optically coupled etalons overlap at only a single passband. Directly optically coupling the first and second stacked, optically coupled etalons can result in the filtering of all passbands of multiplexed light except for the one overlapping passband. Therefore, optically mismatching stacked, optically coupled etalons can be used as an optical filter to select only a single passband.
In accordance with another aspect, one or more of the etalons, or one or more of the etalons of the stacked, optically coupled etalons, preferably each etalon, comprises a bulk optic having first and second parallel, selectively transparent surfaces. The bulk optic comprises a solid optically transparent body (at the wavelengths of interest) and optionally comprises a wedge correcting coating (referred to here generally as a xe2x80x9cwedge coatingxe2x80x9d) and/or a thickness-adjustment layer on at least one of the two surfaces of the optically transparent body. The wedge coating, further described below, establishes high precision parallelism of the selectively transparent surfaces of the etalon. The thickness of the bulk optic (including any wedge coating), i.e., the dimension between the selectively transparent, parallel surfaces, defines the cavity spacing. Preferably, the bulk optic, including the wedge coating, will typically have an optical thickness equal to an integral number of half waves for the wavelength of interest. In preferred embodiments the selectively transparent surfaces are thin film mirror coatings comprising, for example, a film stack of alternating high and low refractive index oxides or a metal thin film in accordance with known thin film technologies. One skilled in the art, given the benefit of this disclosure, will be able to construct optically mismatched etalons and stacked, optically coupled etalons by, for example, varying the thickness of the bulk optic, varying the materials of the bulk optic, varying the properties and compositions of the thin films, etc.
If a wedge coating is used, the thickness of the wedge coating varies progressively across the etalon. That is, the thickness of the wedge coating, viewed in cross-section in at least one plane orthogonal to the parallel, selectively transparent surfaces of the etalon, has a thickness that increases (or decreases in the opposite direction) continuously, typically approximately linearly, to compensate for non-parallelism, or xe2x80x9cwedgexe2x80x9d, in the underlying body of the bulk optic. As described further below, the bulk optic can be diced from a wafer on which a wedge coating and the two thin film coatings have been deposited by magnetron sputtering, ion beam sputtering or other known deposition techniques. Preferably, surface polishing is performed to first polish the wafer, for example, a silica wafer suitable for optical filter production, to parallelism within 1 to 2 arc seconds and wavefront error of less than {fraction (1/50)} (2.0%) of a wave at the wavelength of interest. For an etalon intended for use as one of multiple stacked etalons in an optical element in an optical telecommunication system, the wavefront error will preferably be less than {fraction (1/50)} of a wave at 1550 nm. Low wavefront error can be understood in this context to mean that the thickness of the bulk optic, i.e., the distance between the two opposite surfaces of the bulk optic, is substantially linearly variable and, hence, controllable or correctable by a wedge coating in accordance with the present disclosure.
In accordance with preferred embodiments, the wedge coating is deposited onto the optically transparent body of the bulk optic by physical vapor deposition, i.e magnetron sputtering or ion beam sputtering in a vacuum chamber, with the bulk optic (alone or as part of a larger substrate, such as a typical 6 inch or larger substrate wafer used in the production of optical filters) not spinning during deposition. Preferably the coating is a low defect coating to allow optical contact bonding or other optical coupling of the stacked etalons. Deposition can be otherwise in accordance with known techniques, whose applicability and manner of implementation will be within the ability of those skilled in the art given the benefit of this disclosure. The substrate is oriented at an angle to the target or otherwise arranged to receive progressively different deposition rates from one edge to the opposite edge. The target is preferably an elongate source to provide a coating with a roughly linear profile. Preferably, the target is tangential to the circumference of the substrate and is about four times as large as the substrate, e.g. for a 6 inch diameter substrate the target can be about 20 inches long. The thinnest point of the substrate is positioned closest to the target or otherwise oriented or favored to have the fastest rate of deposition. Conversely, the thickest point is positioned or oriented to have the lowest deposition rate. Thus, the sputtered material will deposit fastest and, therefore, the most heavily, i.e., the thickest, where the bulk optic was thinnest, with progressively thinner deposition toward the area where the least was wanted. Since the substrate is not spinning and is oriented or arranged as just described, the thickness of the resulting wedge coating will change progressively (hence the term xe2x80x9cwedgexe2x80x9d), with the change in the thickness of the wedge coating being opposite that of the underlying body. The net effect is that the thickness of the bulk optic is substantially uniform over all or a large portion of its area. Where the bulk optic is prepared in the form of a typical substrate wafer, thin films can then be deposited to complete the etalon (subject to any further production or packaging steps etc.) in the same or a different sputter deposition chamber. Deposition of a wedge coating and thin films on another, second surface of the bulk optic may be performed in accordance with the embodiments described here.
In accordance with another aspect, one or more of the etalons contained within stacked, optically coupled etalons, most preferably all of the stacked etalons, each comprises a thickness-adjustment layer of substantially uniform thickness on at least one of the two surfaces of the optically transparent body. As used herein, substantially uniform thickness means the thickness of the layer across the surface of the bulk optic is approximately constant. The thickness-adjustment layer, further described below, establishes the desired thickness of the bulk optic cavity of the etalon. One skilled in the art, given the benefit of this disclosure, will recognize that the thickness of the thickness adjustment layer may be varied to achieve etalons having different passband responses suitable for optical mismatching. The bulk optic is a solid, optically transparent (at the wavelengths of interest) body whose thickness, i.e. the dimension between the selectively transparent, parallel surfaces, including the thickness-adjustment layer, defines the cavity spacing. In particular, the bulk optic, including the thickness-adjustment layer, will typically have an optical thickness equal to an even numbers of QWOTs at the same or all points for the wavelength(s) of interest. In preferred embodiments, the selectively transparent surfaces are Fabry-Perot thin film coatings comprising, for example, a film stack of alternating high and low refractive index oxides or a metal thin film in accordance with known thin film technologies. Preferably, the thin film coatings comprise a continuous uniform thickness metal film. As described further below, the bulk optic can be diced from a wafer on which a thickness-adjustment layer and the two Fabry-Perot thin film coatings have been deposited by magnetron sputtering, ion beam sputtering, or other known deposition techniques. Preferably, surface polishing is performed to first polish the wafer, for example, a silica wafer suitable for optical filter production, to parallelism within 1 to 2 arc seconds and wavefront error of less than {fraction (1/50)} (2.0%) of a wave at the wavelength of interest.
In accordance with certain embodiments as disclosed above, the stacked, optically coupled etalons, suitable for use in the optically mismatched stacked, optically coupled etalons, may be bonded to each other. Each etalon comprises any or all of the components listed above including, but not limited to, a bulk optic, optionally comprising a wedge coating and/or a thickness-adjustment layer, and thin film mirror coatings on the surfaces of the bulk optic. Between adjacent etalons may be a layer of bonding material that acts to optically couple the etalons together. The thickness of the bonding layer is preferably equal to an odd number of QWOTs. This bonding layer may be an adhesive, such as an epoxy (i.e. EPO-TEK 353ND from Epoxy Technology, Inc. in Billerica, Mass.), an adhesive optionally having a monolayer of small beads embedded in the adhesive to facilitate precise spacing of the adjacent etalons, fritted glass or any other composition or material that may be deposited to an odd number of QWOTs and is capable of bonding the etalons together. One skilled in the art will recognize, given the benefit of this disclosure, that the bonding material described here may be used to directly optically couple the etalons and stacked, optically coupled etalons.
In accordance with certain preferred embodiments, the etalons of the stacked, optically coupled etalons are joined by depositing an optically transparent fritted glass layer on a surface of etalons. The fritted glass is preferably deposited using a vapor deposition process to a controlled thickness. The fritted glass layer may be deposited onto substrate wafers as disclosed above or onto individual etalons. Two or more wafers can be stacked or, alternatively, individual coupons or individual etalons can be stacked after being coated with fritted glass. After being placed in physical contact, the etalons are heated to the melting or softening point of the fritted glass, bonding the stacked etalons together. Preferably, after the etalons are joined, the thickness of the fritted glass layer between them is equal to an odd number of QWOTs. One skilled in the art will recognize, given the benefit of this disclosure, that this process may be repeated using additional etalons to increase the number of etalons that are joined and stacked.
In accordance with certain preferred embodiments, the etalons are joined using xe2x80x9cfeet.xe2x80x9d As used herein, xe2x80x9cfeetxe2x80x9d are projections from opposite surfaces of adjacent etalons. xe2x80x9cFeetxe2x80x9d can be formed of fritted glass or other suitable materials able to provide, a structurally stable stacking of the adjacent etalons to each other. After adjacent etalons are positioned in contact, heat can be applied to sufficiently melt or soften the xe2x80x9cfeetxe2x80x9d so as to join the etalons to each other. Preferably, after joining the etalons, the distance between the etalon surfaces is equal to an odd number of QWOTs.
In accordance with certain preferred embodiments, adjacent etalons are joined using a layer of epoxy or other suitable adhesive. The adhesive can be applied using any suitable technique. The adhesive may be deposited onto wafers, coupons, or onto individual etalons. Two or more wafers, two or more coupons, or individual etalons are stacked in physical contact after the adhesive is applied. After being placed in physical contact, the adhesive layer bonds the etalons forming stacked, optically coupled etalons. Preferably, after the etalons are joined, the thickness of the epoxy layer is equal to an odd number of QWOTs. One skilled in the art will recognize, given the benefit of this disclosure, that this process may be repeated using additional etalons to increase the number of etalons that are coupled and stacked. In accordance with certain preferred embodiments, the etalons are joined using adhesive comprising optically transparent beads sized to function as a monolayer to assure a controlled spacing between the etalons. The small bead/adhesive mixture may be deposited onto wafers, coupons or onto individual etalons. One or more wafers, coupons, or etalons are placed in physical contact after being coated with the bead/adhesive mixture. After being placed in physical contact, the bead/adhesive mixture bonds the etalons forming stacked, optically coupled etalons having a uniform separation. Preferably, after the etalons are joined together, the thickness of the bead/adhesive layer is equal to an odd number of QWOTs. One skilled in the art will recognize, given the benefit of this disclosure, that this process may be repeated using additional etalons to increase the number of etalons that are coupled and stacked.
In accordance with preferred embodiments, the etalons may be coupled together and/or optically mismatched using optical contact. To accomplish optical contact bonding, adjacent etalons are placed in physical contact with each other. Etalons are prepared as described above. The etalons are then placed into physical contact and, while not wishing to be bound by theory, it is presently understood that adjacent etalons in optical contact with each other are held together by electrostatic forces between the contacting surfaces of the etalons. For certain preferred embodiments, the selectively transparent thin film mirror coatings of the stacked etalons are formed of alternating films of high refractive index material and low refractive index material. Preferably, the outermost film of the mirror coating of one etalon is the same (i.e., either low or more preferably high refractive index) as the outermost film of the mirror coating of the adjacent etalon. The spacer or transition layer between them preferably has the opposite refractive index. Thus, if the two outermost films facing each other are high refractive index, then an air spacer or other low refractive index material preferably fills the odd QWOT space between the two etalons. Likewise, if the mirror coatings finish with low refractive index materials, then the joining transition layer between the two etalons should have a high refractive index. Alternatively, the thickness of the outermost layer of the thin films may be reduced such that, when combined together they form a joining transition layer of refractive index opposite to that of the next film on either side. In other preferred embodiments, the etalons may be held in a stacked position using a coupling member such as a metal jacket, sleeve, etc. Should a coupling member be used, any space that exists between the etalon surfaces is preferably equal to an odd number of QWOTs and may be filled with air or other suitable material.
In accordance with another aspect, additional cavities can be deposited on either surface of the bulk optic etalons by suitable deposition techniques, such as, for example, ion beam sputtering, magnetron sputtering, etc. That is, additional cavities can be formed on a bulk optic etalon. More specifically, following deposition of a suitable transition layer overlying the thin film stacks on either surface of the bulk optic etalon, a cavity can be deposited by, for example, sputtering a first suitable thin film coating or stack, such as a sequence of alternating H/L/H film, followed by deposition of a cavity film, followed by deposition of a second thin film coating or stack which preferably is substantially identical to the first thin film coating. Similarly, yet one or more such thin film etalons can be deposited thereafter employing suitable odd QWOT transition layers between, in accordance with thin film principles well known to those skilled in the art. Thus, the advantage of improved optical performance characteristics of a multi-cavity device can be achieved, incorporating a bulk optic etalon of the type disclosed above together with additional thin film etalons unitary therewith by depositing such thin film etalons thereon using sputtering or other suitable deposition techniques. Any suitable materials disclosed above, and other suitable materials known to those skilled in the art, may be used for the thin films of the thin film etalons including but not limited to Ta2O5, ZrO2, TiO2, Al2O5, SiO2, and MgF. The multi-cavity deposition product, described above, is suitable for use, generally together with or in place of the single cavity embodiments of the bulk optic etalons disclosed here.
In accordance with a method aspect, optical filter elements are used to select a single passband from multiplexed light. The optical filter element may comprise numerous configurations, such as those discussed above. For example, the optical filter element may comprise etalon/etalon, etalon/stacked etalons, stacked etalons/stacked etalons, etc. The optical filter elements, including but not limited to the etalons and stacked, optically coupled etalons, may be housed within an optical system. The optical system includes a source of multiplexed light or multiplexed signals divided into multiple channels 1-n, where n is the total number of channels. The multiplexed signals are incident upon the optical filter element, e.g. light entering through a port in an optical fiber is incident on a surface of the optical filter element. The light passes through a first surface of the optical filter element, and certain passbands are passed in accordance with the passband response of the optical filter element or the first portion of the optical filter element, e.g. the first etalon. Passbands that are not passed by the first component of the optical filter element may be reflected and can enter a second port in the optical system. The first output signal from the first component of the optical filter element, e.g. the first etalon, is incident on the first surface of the second component of the optical filter element, e.g. a second etalon. A second output signal, passed by the second optical component, can be received at an additional port, e.g. a third port, for detection. Preferably the second output signal comprises only a single passband. That is, the second output channel comprises only the overlapping passband of the optical components of the optical filter element. One skilled in the art, given the benefit of this disclosure, will recognize that the components of the optical system may be arranged in numerous configurations. For example, the surfaces of the optical filter elements may be in direct and continuous contact, or the surfaces may be separated by an air space. Preferably, the longitudinal axis of the thin film coatings of the etalons, and stacked, optically coupled etalons, are perpendicular to the direction of the optical path.
Optical telecommunication systems and optical sensors comprising optically mismatched etalons and optically mismatched stacked, optically coupled etalons (and combinations thereof) as disclosed above and the methods disclosed for their production and use will be recognized by those skilled in the art to represent a significant technological advance. Robust optically mismatched etalons and optically mismatched stacked, optically coupled etalons can be produced meeting precise optical performance characteristics, with advantageously low production costs and good production flexibility. In preferred embodiments, the optically mismatched etalons and optically mismatched stacked, optically coupled etalons have the advantageous attributes of small size, simple and potentially inexpensive construction, and good optical performance, including low loss, low polarization dependent loss and polarization mode dispersion, and low chromatic dispersion. Additional features and advantages will be understood from the following detailed description of certain preferred embodiments.